Serial communication networks provide many advantages over other well-known networks such as multipoint, star or mesh networks, the most valuable being a fair distributed polling function which readily supports peer-to-peer communications amongst a large number of stations with a high utilization of the network capacity.
One major drawback of the serial network is its propensity to catastrophic failure when any component of the network fails. The drawbacks of a serial network are somewhat reduced by use of a dual ring communication system. A dual ring communication system is comprised of a primary ring or data path and information is passed in a first direction along the primary ring. No information is passed along the secondary ring until a failure occurs. When a failure occurs the secondary ring may be connected to the primary ring to complete the serial communication network. In such a case information is transmitted along the secondary ring in a second direction opposite that of the primary ring.
Over the years many techniques have been developed to detect and/or isolate faults in serial communication networks. One of the more useful techniques, which is in use today in the IEEE 802.5 Token Ring, is disclosed in U.S. Pat. No. 3,564,145. This technique known as beaconing identifies a station, by its address, immediately downstream of a failed network component or station. In a static network (e.g. one in which the network topology is fixed or known) corrective action can be taken to bypass or fix the failed network component.
Another technique (Dual Ring Reconfiguration) has proven very useful in the isolation of faults in a serial network, thereby providing complete or partial network operation following the failure of a network component. This technique employs the dual serial rings described above which may be converted to single ring via switching means to thereby bypass a failed network component. The patents listed below disclose a variety of Dual Ring Reconfiguration implementations:
U.S. Pat. No. 3,519,750 PA0 U.S. Pat. No. 3,876,983 PA0 U.S. Pat. No. 4,009,469 PA0 U.S. Pat. No. 4,354,267 PA0 U.S. Pat. No. 4,390,984 PA0 U.S. Pat. No. 4,527,270 PA0 U.S. Pat. No. 5,538,264 PA0 U.S. Pat. No. 4,594,790 PA0 U.S. Pat. No. 4,709,365 PA0 U.S. Pat. No. 3,458,661 PA0 U.S. Pat. No. 4,035,770 PA0 U.S. Pat. No. 4,048,446 PA0 U.S. Pat. No. 4,245,343 PA0 U.S. Pat. No. 4,763,329
The patents listed below disclose a variety of manual and automatic techniques for bypassing a failed network component in a single ring serial network:
While all of the techniques described above are useful either by themselves or in combination, they may have difficulty in providing fast, efficient or complete restoration of communications in a serial ring network following failures of all kinds.
Modern serial networks such as the IEEE 802.5 Token Ring Network generally employ many (up to several hundred or more) ports located throughout an establishment. Many of these ports are not utilized or are connected to inactive stations. In addition, stations (each of which includes a unique identity or address) are frequently moved from one port to another for the convenience of the operator.
In view of the mobility of the stations and the large number of ports which have no or inactive stations connected, the station identity or address accompanying a beacon message provides little information to locate the geographic position of the failed network component.
The technique (Next Active Upstream Neighbor) disclosed in U.S. Pat. No. 4,507,777 is very useful in managing fault recovery in serial networks; however, the sequential station identities or addresses derived from this technique do not provide sufficient network topology information to accurately pinpoint the physical location of the failed component. For example, two adjacent active stations may be separated on the physical network by a number of non-connected or inactive ports. Thus, knowing that station X detected a failure and that station C preceded X does not physically locate a particular faulty component.
U.S. Pat. No. 5,132,962 describes a reconfiguration unit capable of fault isolation and bypass in a dual ring communication system. While this unit fully implements the IEEE 802.5 fault isolation and bypass procedures, it requires three adapters to interface to the primary and secondary rings of the dual ring communication system. These adapters are the single most expensive component of the reconfiguration unit and, therefore, a unit requiring three such adapters may be impractical in certain applications where the high degree of error recovery cannot justify the cost of such reconfiguration units.
Because many existing dual ring networks have been implemented with the technology described above any reconfiguration unit design should be compatible with such networks. Furthermore, to assure compatibility with future designs of reconfiguration units it would be desirable that any reconfiguration unit design or method of fault isolation and bypass be compatible with a network protocol such as the IEEE 802.5 Token Ring standard.